Tuesday, October 16, 2012

Designing a Grid-Tie Inverter Circuit

A grid tie inverter works quite like a conventional inverter, however the power output from such inverter is fed and tied with the AC mains from the utility grid supply. As long as the mains AC supply is present, the inverter contributes its power to the existing grid mains supply, and stops the process when the grid supply fails.
The concept is indeed very intriguing as it allows each of us to become an utility power contributor. Imagine each house getting involved in this project to generate overwhelming amounts of power to the grid, which in turn provides a passive income source to the involved residences. Since the input is derived from the renewable sources, the income becomes absolutely free of cost.

Making a grid tie inverter at home is considered to be very difficult as the concept involves some strict criteria to be observed, not following may lead to hazardous situations.

The main few things that must be observed are:

The output from the inverter must be perfectly synchronized with the grid AC.

The output voltage amplitude and frequency as mentioned above must all correspond with the grid AC parameters.

The inverter should switch OFF instantly in case the grid voltage fails.

In this post I have tried to present a simple grid-tie inverter circuit which according to me takes care of all the above requirements and delivers the generated AC into the grid safely without creating any hazardous situations.

Let's try to understand the proposed design (exclusively developed by me) with the help of the following points:

Again, as usual our best friend, the IC555 takes the center stage in the entire application. In fact only because of this IC the configuration could become apparently so very simple.

Referring to the circuit diagram, the IC1 and IC2 are basically wired up as a voltage synthesizer or in a more familiar terms a pulse position modulators.

A step down transformer TR1 is used here for supplying the required operating voltage to the IC circuit, and as well as for supplying the synchronization data to the IC, so that it can process the output in accordance with the grid parameters.

Pin#2 and pin#5 of the both the ICs are connected to the point after D1, and via T3 respectively, which provides the frequency count and amplitude data of the grid AC to the ICs respectively.

The above two information provided to the ICs prompts the ICs to modify their outputs at the respective pins in accordance with these information.

The result from the output translates this data into well optimized PWM voltage that's very much synchronized with the grid voltage.

IC1 is used for generating positive PWM, while IC2 produce negative PWMs, both work in tandem creating the required push pull effect over the mosfets.

The above voltages are fed to the respective mosfets, which effectively converts the above pattern into a high current fluctuating DC across the involved step up transformer input winding.

The output of the transformer converts the input into a perfectly synchronized AC, compatible with the existing grid AC.

While connecting the TR2 output with the grid, connect a 100 watt bulb in series with one of the wires. If the bulb glows, means the ACs are out of phase, reverse the connections immediately and now the bulb should stop glowing ensuring proper synchronization of the ACs.

WARNING: THE IDEA IS BASED SOLELY ON IMAGINATIVE SIMULATION, VIEWER DISCRETION IS STRICTLY ADVISED.You would also want to see this simplified design
After receiving a corrective suggestion from one of the readers of this blog Mr. Darren and some contemplation, it revealed that the above circuit had many flaws and it wouldn't actually work practically.

The revised design is shown below, which looks much better and a feasible idea.

Here a single IC 556 has been incorporated for creating the PWM pulses.
One half of the IC has been configured as the high frequency generator for feeding the other half IC which is rigged as a pulse width modulator.

The sample modulating frequency is derived from TR1 which provides the exact frequency data to the IC so that the PWM are perfectly dimensioned in accordance with the mains frequency.

The high frequency makes sure the output is able to chop the above modulation information to precision and provide the mosfets with an exact RMS equivalent of the grid mains.

Finally, the two transistors make sure that the mosfets never conduct together rather only one at a time, as per the mains 50 or 60 Hz oscillations.

Parts List

R1,R2,C1 = select to create around 1 kHz frequency

R3, R4,R5,R6 = 1K

C2 = 1nF

C3 = 100uF/25V

D1 = 10 amp diode

D2, D3, D4, D5 = 1N4007

T1, T2 = as per requirement

T3, T4 = BC547

IC1 = IC 556

TR1, TR2 = as suggested in the previous section design

The above circuit was analyzed by Mr. Selim and he found some interesting flaws in the circuit. The main flaw being the missing negative PWM pulses of the AC half cycles. The second fault was detected with the transistors which did not seem to isolate the switching of the two mosfets as per the fed 50 Hz rate.

The above idea was modified by Mr. Selim, here are the waveform details after the modifications. modifications:

Waveform Image:

CTRL is the 100 Hz signal after the rectifier, OUT is from PWM from both halve waves, Vgs are the gate voltages of the FETs, Vd is the pickup on the secondary winding, which in sync with CTRL/2.

Disregard the frequencies as they are incorrect due low sampling speeds (else it gets too slow on the ipad). At higher sampling freqs (20Mhz) the PWM looks quite impressing.

To fix the duty cycle to 50% at around 9kHz, I had to put a diode in.

Regards,

Selim

For enabling the detection of the negative half cycles, the control input of the IC must be fed with both the half cycles of the AC, this can be achieved by employing a bridge rectifier configuration.

Here's how the finalyzed circuit should look according to me. The transistor base is now connected with a zener diode so that would hopefully enable the transistors to isolate the mosfet conduction such that they conduct alternately in response to the 50 Hz pulses at the base T4.

Recent Updates from Mr. Selim

Hello Swags,

I keep reading your blogs and continue experimenting on the breadboard.I have tried the zener-diode approach (no-luck), CMOS gates and, much better, op-amps worked best. I've got 90VAC out of 5VDC and 170VAC from 9VDC at 50Hz, I believe it's in sync with the grid ( can't confirm as no oscilloscope). Btw the noise goes if you clamp it with a 0.15u cap. on the secondary coil.

As soon as I put a load on the secondary coil, it's voltage drops to 0VAC with only a slight increase in input DC amps. The Mosfets don't even try to draw more amps. Perhaps some mosfet drivers like IR2113 (see below) could help?

Although in high spirits, I feel that PWM might not be as straight forward as hoped. It definitely is good to control torque on dc motors at low pwm freqs. However when the 50 Hz signal gets chopped at higher freq, it for some reason looses power or the PWMd mosfet can't deliver the needed high amps on the primary coil to keep the 220VAC going under load.

I've found another schematic which is very closely related to yours, except PWM. You might have seen this one before.

The link is on http://www(dot)electro-tech-online(dot)com/alternative-energy/105324-grid-tie-inverter-schematic-2-0-a.html

The power handling circuit is an H drive with IGBTs (we could use mosfets instead). It looks like it can deliver the power across.

It looks complicated but actually is not too bad, what do you think? I will try to simulate the control circuit and let you how it looks.

Regards,

Selim

Sent from my iPad

Some very interesting modifications and information were provided by Miss Nuvem, one of the dedicated readers of this blog, let's learn them below:

Hello Mr. Swagatam,

I am Miss Nuvem and I'm working in a group that is building some of your circuits during an event about sustentable living in Brazil and Catalonia. You have to visit some day.

I've been simulating your Grid-Tie Inverter Circuit, and I'd like to suggest a couple of modifications to the last design that you had on your post.

First, I was having problems where the PWM out signal (IC1 pin 9) would just blank out and stop oscillating. This was happening whenever the Control voltage at pin 11 would go higher than the Vcc voltage due to the drop across D4. My solution was to add two 1n4007 diodes in series between the rectifier and the control voltage. You might be able to get away with just one diode, but I am using two just to be safe.

Another problem I was having was with the Vgs for T1 and T2 not being very symmetric. T1 was fine, but T2 was not oscillating all the way up to Vcc values because whenever T3 was on, it was putting 0.7V across T4 instead of letting R6 pull up the voltage. I fixed this by putting a 4.7kohm resistor between T3 and T4. I think any value higher than that works, but I used 4.7kohm.

I hope this makes sense. I am attaching an image of the circuit with these modifications and the simulation results that I am getting with LTspice.

We'll be working on this and other circuits for the next week. We will keep you updated.

hi swagatam this is kurush karanjia here. This circuit of yours can be very help full in the PCU i am trying to make by synchronizing inverter output to the AC mains output and then suppling to loads. So please help and tell me how can i implement this circuit in the way i want.

I have assumed the IC to handle a few things which have been mentioned in the article, if that happens correctly rest of the things will automatically shape up.

Synchronizing phase amplitude and frequency are the only things which are important. And also the inverter should switch OFF when mains is not available. All these things have been taken are of in the above circuit.

ok no problem i will take all possible precautions and will use a mcb before this entire deviceAnd i will be directly using the inverter output so which part of the circuit should i eliminate and where to collect the load

i think by mistake you have replied to this question in another post but its ok i got what you said but my question now is that i am gonna use the inverter output directly so will need to eliminate some portion of this circuit so what portion should i eliminate...?

The meter will run in the reverse if being fed.I have seen grid tie inverter and they have a complex a/c synchronization ic circuit and they work in a range of the ac voltage.Does this circuit also have a working voltage for the grid a/c and also frequency since frequency might drop due to heavy loads being used

As per my assumptions, the above circuit derives the frequency and the amplitude information from the existing AC grid input and processes the circuit output accordingly, therefore the circuit should be producing perfectly synchronized AC back to the grid.

Both the ICs should produce waveforms in tandem by translating both the halves of the grid AC cycles, therefore BC547 is added so that the bottom IC is able to translate the negative half cycles into the required PWM.

T1 and T2 are same.

"sources" are made common and connected to ground, and the drains are connected to the transformer taps.

First test the inverter separately, do not connect the output of the inverter to the grid, instead connect the output to a 100 watt bulb, next connect a DC power source may be a battery 25AH 12Vand to the shown points, check the bulb illumination.

TR1 must be connected to the AC mains as shown for the above testing.

If the bulb illuminates fully then we can proceed further.

WARNING: THE SUGGESTIONS ARE BASED SOLELY ON MY IMAGINATIVE SIMULATION.....

While connecting the TR2 output with the grid, connect a 100 watt bulb in series with one of the wires. If the bulb glows, means the ACs are out of phase, reverse the connections immediately and now the bulb should stop glowing ensuring proper synchronization of the ACs.

Hi Swagatam,Been following this post for a while now and cannot beleive the amount of novice`s trying to build this un-tested project with no idea of what the components are doing. Do some research guys before you kill yourselfs.

Looked at the circuit myself and I can understand the half wave entering the chip at pin 5 which will follow the mains frequency but cannot see what you are trying to do with pin 2 as this is also at the same frequency.

You state that the chip is wired as a pulse position modulator which would be fine if it had another higher frequency either injected into pin 2 plus a timing cap or simply ocilating the chip at a higher frequency.

The tall and short of this is where is the higher frequency derived from other than the 50/60Hz that is injected ?.

Having done some more research about the 555 timer chip it seems you would have problems with the duty cycle only operating from 10%-90% which could cause both outputs to be on at once.

I am thinking that a better aproach to this problem would be op-amps.

Using something like a LM324N, 2 op-amps creating a high frequency saw-tooth and fed to the neg of the remaining two op-amps then half waves (50/60Hz) fed to the positive of the op-amps creating two seperate pwm outputs.

Maybe this would be a easier option and only one chip to buy. All off the top of my head at the moment !!!.

Yes surely opamps can be used also as suggested by you, but since I have already done some hard work in studying IC555, in my new design I employed them yet again, and hopefully eliminated the previous issues in this design.

I have presented the revised design in the above article, I hope this time you won't have any trouble with it :)

No it's not correct, the sources should be made common and connected to ground and the drains should go to the taps of the transformer...I'll have to correct the diagram one more time...I'll do it soon hopefully.

Hi Swagatam, great project, I tried the schematic on a breadboard and it worked so far. I changed T2 drain/sorce pins, used IRF740PbF Mosfets and 15VA toroidal TR2. I used a bench power supply as a DC source and went up to 5VDC/0.04A, which produced a 80VAC. Didn't go any higher as the noise levels went up (around 0.8kHz). Also, I did not connect TR2 to TR1 as I have no oscilloscope to confirm wave form and phase. R1 was 6.8k, R2 68k, C1=10n and R7 was 1k (it wasn't mentioned). In your prev. schematics you used capacitors beteween R5 and T1 and (R6 and T2). Could this filter out the 0.8-0.9kHz noise from the transformer TR2? Also, would you suggest that running the timer from a stabilised power source like 78L12 could reduce the noise while feeding in the control frequency through an optical coupler? Could an optical coupler provide both frequency and amplitude data? Thanks for your advise. Br, Selim

Those who are asking most of these newbie questions do not know enough to do this safely. Many dangers lurk here, electrocution, fire, damage to connected equipment, etc. Great project, but seriously not one to undertake without a good understanding of whats going on!

Hi Swagatam, could you please help me understand the function of T3/4. I know it's important that both FET-gates should not be positive at the same time. Using breadboard, and iCircuit simulator, I always got the same results. Only one FET was operating (10V pulse at gate) and the other one at 0.8V pulse at gate, not enough to open the gate (>2V) . So I tried using 3 NOR gates of a 4001 and it switched sides at 50Hz. Was this the purpose of T3/4 or did you put them in to prevent the FETs T1/2 conducting at the same time only? BTW, can I contact you by email?Regards,Selimps disregard prev msg

On iCircuit (iPad) and on the breadboard, I get at the gates: T1=+12.2V, and T2=+0.845V PWM-signals, same phase, not alternating between T1/2.

So effectively only one side of the primary winding coil (6V-0-6V) is energised. Leaving T3/4 running on the breadboard, using 2.5VDC/0.05A input, I get around 90VAC at 820-870Hz (wobbling at around 50Hz, I think).

Using NORs from 4001, I can get it alternated in sync between T1/2, now here comes the next question. On iCircuit, I noticed that the positive wave gets PWM modulated properly where as the negative wave (or the missing bit/flat line between 2 positive waves) is only PWM modulated as a series of short pulses (no width/no power). The output would be distorted, such as larger positive waves vs. smaller negative waves. Is there a way to capture the negative side as well? Maybe another NE555/6 could do this job using the same 1kHz chopper.

Please keep this project up. It seems like it could work well. Regards

Hi Swagatam, I managed to pickup both waves, and PWMed them and then run through 4013 to divide the 100Hz by two. The results look good on iCircuit. Both wave halves are PWMed at 9kHz and alternated on T1/2 evenly. The secondory winding phase in-sync with control input. I'll email you the pics later today. Two 4001 can be used iso 4013, but would require more wiring on the breadboard is needed. I think two 555s could do a much better job. I haven't figured out how to feed a 555 with a negative wave yet.Regards,Selim

I got a little concern about what would happen if this circuit was disconected from the mains but still getting a DC input from say a solar panel/turbine. The way i see it is that TR2 will keep trying to feed TR1 and the whole circuit will go out of control resulting in the end of the plug being live. A normal mains failure will most likely drown TR2 out due to not being able to feed a whole city !!.Hope i am missing something here.

That should be OK, the idea is to keep the transistor conductions well isolated, and in an alternate manner.....the zener diode makes sure that the transistor conductions do not overlap (as far as I thing).

I have a solar panel of 150W, 24V. It is currently hooked up by a charge controller and a lead acid battery. I just need an inverter which directly converts pv supply (24v dc) into 220v ac, without having a battery.I only need to run an 80w fan on it. Can you please help me in this regard.

The second last circuit is the finalized design, but please note that it's only the concept which I have tried to produce, the person who intends to build it should have a sound knowledge regarding electronics and inverters, only then will he or she be able to implement the design correctly for practical feasibility.

If your not an expert in the field please don't even think of making such complex circuits, you will end up wasting a lot of money and time.

hi,i have solar panel 58v amorfni 128w 4 pieces do i go in serial 220v then adjust your circuit for it (220vdc) or do i make in paralel 4 x 58v then adjust your circuit for 58v what do you mean by that two cobination?

Hi,the above design is yet to be verified, so I would recommend you to first test the second last design shown above, if it performs as per the expectations, you could go on upgrading it from there....

By the way, putting solar panels in series makes more sense and is always more efficient.

I am Miss Nuvem and I'm working in a group that is building some of your circuits during an event about sustentable living in Brazil and Catalonia. You have to visit some day.

I've been simulating your Grid-Tie Inverter Circuit, and I'd like to suggest a couple of modifications to the last design that you had on your post.

First, I was having problems where the PWM out signal (IC1 pin 9) would just blank out and stop oscillating. This was happening whenever the Control voltage at pin 11 would go higher than the Vcc voltage due to the drop across D4. My solution was to add two 1n4007 diodes in series between the rectifier and the control voltage. You might be able to get away with just one diode, but I am using two just to be safe.

Another problem I was having was with the Vgs for T1 and T2 not being very symmetric. T1 was fine, but T2 was not oscillating all the way up to Vcc values because whenever T3 was on, it was putting 0.7V across T4 instead of letting R6 pull up the voltage. I fixed this by putting a 4.7kohm resistor between T3 and T4. I think any value higher than that works, but I used 4.7kohm.

I hope this makes sense. I am attaching an image of the circuit with these modifications and the simulation results that I am getting with LTspice.

Dear SirI have to Sun grid tie inverters. One input 10-30 volts and other 22-60.For my wind turbine I have conclude the best range is 15-40 volts. Is suitable to change them to thiese range? If yes how?.I appreciate your replayBest regardsJose Castanho - PortugalEmail: castanho.jose@gmail.com

Ryner here, I'm a student doing GTI for my Final Year Project. I would like to raise a more theoretical question: why is there a need to synchronize the output of the GTI with the grid? What would happen if it is not synced?

since the involved current is varying from positive to negative every split second, if the two counterparts are not synchronized, would result in possible clashing of their opposite cycles in their AC phases which would in return result either in short circuiting of the power or one of the counterparts dragging the power toward lower levels thereby heating up the inverter devices.That's why perfect synchronization becomes the ultimate crucial thing in GTI systems.

Ryner here again, sorry to trouble you with another question, but from what I understand, the output of the GTI has to be channeled back to the grid. Does this mean that my output from the GTI should have a slightly higher amplitude compared to that of the grid? (by theory, a potential difference is required for current to flow) On that note, are we transmitting energy in terms of voltage or current? Thank you in advance!

Ryner here once again. How exactly do I get the output waveform to synchronize with the grid? Would I be able to do it with a Phase Lock Loop (PLL) Chip? Is there a simpler way to do the synchronization process? Thank You!

I am doing my final year project with renewable energy, i prefer doing this circuit. I am doing it for a grid of 230V what must be my transformer TR2's ratings? Can i have the full final circuit?Please help, Thank you in advance.

the DC input could be from a battery or any regulated source. yes we'll have to add a comparator sensing stage for ensuring that the battery supply is cut off as soon as its voltage drops below the input DC ripple voltage from TR1 at pin#11 of the IC

Dear Dharamveer, if the DC input to the circuit is maintained at some constant, safe higher level say at 15V for a 12V system, the circuit will automatically take care of all the other factors associated with the mains supply (according to me)

hi Swagatam Majumdarcan I connect directly to the solar dc source. can save a lot of costs.but I have one problem, if larger loads feeds from tie circuit .circuit continues to operate or shut down thanks

yes you can connect it directly to a solar panel, however the supply to the ICs must not exceed 15V.The circuit output will follow the grid voltage proportionately...if the grid voltage drops so will the output from the inverter and vice versa...so with larger loads the output will adjust in accordance with the grid voltage.

OK sorry, i confused it with the other circuit...the frequency will is locked with grid frequency so it will be always 50Hz at the trafo output, the 1khz is for generating the PWMs, it's not relevant to the output frequency.

Sure...the only missing with me is the 68k i hope i can find this afternoon and try to blow in the evening..anyway i let you know once it work smoothly..also i found some diagram from the net but not sure with it...but too many blogs says it works???

I try.. Its busted...any way its fine...im waiting for my scope..its hard to check all of this..!!! I will try your other design and let see we were going to blow that one aswell...i will inform you what ever i found...but this one...my 10amps transformer get hot aswell..its reallly busted.i want to follow only what is in the doagram to check if its actually working but i put all the nessisary precaution..!!!!

I have made ​​such a design above. but I do not see the output of the R5 and R6 using osiloscop. and I have tried to synchronize between the inverter and generator 1 phase using a 30 watt lamp. I see there is a change from the original load dim lights become bright.

when there is no load voltage and frequency phenomenon after the sync is 220 volts to 210 volts and 50Hz be 49.5 Hz. but the mosfet is used even though I use a cooler heat. what should I do to reduce the heat? and what will not be a burden inverter generator?

to make the Grid free from an AC mains potential which could otherwise kill or hurt an electrician who may be possibly at work across a faulty grid line without any caution, assuming the line to be switched OFF and safe.

Please Share!

You all are Awesome

About Me

I am an electronic hobbyist, inventor, a freelance electronic circuit designer, manufacturer. I am also an avid publisher, I enjoy posting electronic related articles in my website: http://www.homemade-circuits.com/ where the interested viewers get the opportunity to read about many innovative electronic circuit ideas, exclusively designed and developed by me.